Global warming is the increase in the average temperature of Earth’s near-surface air and oceans since the mid-20th century and its projected continuation. Global surface temperature increased 0.74 ± 0.18 °C (1.33 ± 0.32 °F) between the start and the end of the 20th century.

Global warming refers to an increase in the Earth’s average surface air temperature. Global warming and cooling in themselves are not necessarily bad, since the Earth has gone through cycles of temperature change many times in its 4.5 billion years. However, as used today, global warming usually means a fast, unnatural increase that is enough to cause the expected climate conditions to change rapidly and often cataclysmically.

Our planet is warmed by radiant energy from the sun that reaches the surface through the atmosphere. As the surface warms, heat energy reflects back toward space; meanwhile, gases in the atmosphere absorb some of this energy and reradiate it near the surface. This is often called the greenhouse effect, named for the way heat increases inside a glass enclosure. In the greenhouse effect around Earth, the atmosphere can be visualized as a blanket that is made thicker by the action of a small amount of water vapor, carbon dioxide, methane, ozone, nitrous oxide, other gases, and soot; it thus holds in more heat, forcing air temperature higher. The scientific term for this action is, in fact, “forcing.”

On an average day, this effect is caused by water vapor and clouds (75 percent) and carbon dioxide (20 percent), with the rest fthe heating caused by other gases. Relatively small additions of carbon dioxide and methane force more heat, and that heat allows the air to hold more water vapor, creating a feedback loop that magnifies the effect. Although water vapor is naturally prevalent in the atmosphere, it does not trap as much heat per molecule as carbon dioxide and methane. Also, water vapor molecules cycle through the atmosphere in only a few days, a brief period compared to the residence time of CO2,which persists for many decades and creates some warming even after as long as three hundred years. Dust and aerosol chemicals in the air cause some cooling (negative forcing); they are also very short lived. Even though the gases are measured only in parts per million (ppm) or billion (ppb), they have been powerfully, and naturally, influencing the Earth’s temperature for millions of years. Without them, instead of an average air temperature of about 58°F (14.5°C), the Earth would be below the freezing point. Life as we know it now would be impossible.

Earth’s temperature is also subject to natural forcing cycles from solar radiation and the movement of the planet around the sun. Scientists think these cycles, which have left a visible signature extending back millions of years, arewhat led to past iceages and the warming that ended them. Currently, we are in a period between major iceages. The last great glaciation, when temperatures were about 10°to 12°F (6°to7°C) cooler than today, began fading away about 18,000 yearsago. The initial transition out of the ice age was unstable,with many rapid temperature shifts. As temperatures warmed, climate was affected.